The present invention generally relates to apparatus and methods for applying powder to objects. More specifically, the invention relates to electrostatic and other powder application apparatus and methods.
Powder coating technology has generally evolved over several years into several different coating techniques performed with various types of coating systems. Generally, a powder, such as a resinous polymer or paint, is initially adhered to an electrically conductive object. This initial coating process typically involves electrically grounding the object and electrostatically charging the powder particles such that the electrostatic attraction causes the powder to adhere to the object ideally, in most applications, with a uniform coating thickness. This initial powder coating is then cured using heat or other techniques, such as infrared or ultraviolet light. This fully adheres the coating to the object.
Applying powder to internal portions of certain objects presents unique problems. For example, electric motor stators are often shaped cylindrically with inwardly facing slots configured to receive copper windings. There must be an electrically insulating layer between the copper windings and the metal defining the slots of the stator. Therefore, when powder coating techniques are used to provide a layer of insulation on these metal surfaces, the powder must penetrate into the slots of the stator, preferably without saturating the end faces of the stator with powder. Especially when faced with stators having deep slots, it has been difficult to fully penetrate into these slots and provide uniform coatings on the internal stator surfaces. Powder spray guns have been attempted in these situations, but generally impart too much powder velocity and, therefore, blow too much powder out from the slots. On the other hand, parts have been placed in the powder cloud formed by electrostatic fluidized beds. However, this may not provide a uniform coating to internal portions of a part for opposite reasons. That is, in conventional forms electrostatic fluidized bed coaters form a powder cloud within a chamber, but this powder cloud generally moves with very low velocity. For this reason, the powder deposits at the end faces of the stators, but does not penetrate fully into the slots.
For the reasons stated above, as well as other reasons, it would be desirable to provide powder application techniques and apparatus which can more uniformly and effectively coat internal portions of an object, such as a motor stator or other object requiring internal penetration of an object with a more uniform application of powder than provided with past apparatus and techniques.
The present invention generally provides apparatus for applying powder to objects including a first chamber having first and second portions. A powder fluidizing bed is disposed at the first portion of the first chamber and fluidizes a bed of powder to form a powder cloud in the first chamber and moving generally toward the second portion. An opening is disposed at the second portion of the first chamber and directs at least one stream of the powder from the powder cloud out of the first chamber. An object holder is disposed outside the chamber and is configured to hold at least one of the objects at a position for intersecting the stream of powder. In general, the apparatus provides a flow of powder from the chamber which is directed at a greater velocity than the typical velocity of powder within a fluidized bed coating chamber, but less than the velocity from a typical powder spray gun. This allows internal portions of an object, such as a motor stator, to be coated uniformly by forcing the powder through the slots of the stator, or other internal portions of an object, without forcing the powder at such a speed that it will not adhere to the object surfaces due, for example, to electrostatic attraction between the powder and the object.
The apparatus more specifically includes a porous member disposed at a lower portion of the first chamber below the bed of powder. The opening is disposed at an upper portion of the first chamber and having an upper side facing the bed of powder and an opposite lower side. An electrostatic charging device is positioned in an air flow path leading to the lower side of the porous member. A pressurized air inlet directs pressurized air into the air flow path such that the air is charged by the electrostatic charging device and then passes respectively through the lower and upper sides of the porous member and into the bed of powder. The apparatus includes a second chamber having an object inlet and an object outlet. The object holder is positioned in the second chamber and is movable to deliver the object from the object inlet to the object outlet. The object holder preferably includes an arm which rotates to move the object from the object inlet to the object outlet. More preferably, and especially in the case of coating motor stators, the arm extends into the second chamber along an axis and the arm further rotates about that axis to rotate the object within the stream of powder and expose multiple sides of the object to the stream of powder. This can facilitate more uniform penetration of powder into the object, such as into the slots of a motor stator.
As another feature of the preferred apparatus, the opening is adjustable in size to vary physical characteristics of the stream of powder. For example, the velocity and/or the amount of powder in the powder stream may be varied by reducing or increasing the size of the opening in the upper portion of the first chamber. The opening is formed generally at the narrowest region of a converging area inside the first chamber. This, for example, may be accomplished by using a plurality of converging plates which have upper ends generally forming an apex and the opening. At least one of the converging plates is movable with respect to the other to adjust the size of the opening. A height adjustment mechanism may also be coupled with either the first chamber or the object holder, or both, for adjusting the distance between the opening and the object holder. This feature can be helpful to adjust the amount of powder penetration into the object and to adjust for smaller or larger objects being coated or otherwise applied with powder using the same apparatus. As another alternative feature, the opening may further comprise a plurality of separate channels for directing a plurality of separate streams of powder out of the first chamber to thereby increase the velocity of the powder prior to contacting the object. This has been found especially useful when impregnating objects, such as nonwoven substrates, with a powder for various reasons.
A method performed in accordance with the inventive principles generally involves forming a powder cloud within a chamber having first portion with a constricted opening and a second portion; moving the powder cloud generally toward the constricted opening within the chamber; directing a stream of the powder through the constricted opening; and holding the object adjacent the opening and in contact with the stream of powder. As generally discussed in connection with the apparatus described above, the powder cloud is preferably formed and moved by introducing pressurized air through a bed of powder at the lower portion of the chamber and the powder is preferably electrostatically charged, while the object is charged in an opposite manner, such as by electrically grounding the object. In accordance with the preferred method, the object is a motor stator having internal and external portions and the method involves directing the stream of powder through the internal portion and over the external portion of the motor stator to coat the internal and external portions thereof with powder. In accordance with another illustrative example, the object may be a permeable object such as a nonwoven substrate, and the object may be impregnated with powder while holding the object in contact with the stream of powder.